1. Field of the Invention
The present invention relates to an evaporative fuel processing device for an internal combustion engine, and more particularly to an evaporative fuel processing device for an internal combustion engine that stops the supply of a purge gas to the internal combustion engine in accordance with a fuel cut.
2. Background Art
The internal combustion engine for a vehicle is provided with a canister for adsorbing and storing evaporative fuel that is generated within a fuel tank. The evaporative fuel stored by the canister is purged out of the canister by using a negative pressure in an intake path during an internal combustion engine operation. The purged evaporative fuel is diluted with air that is introduced from an atmospheric air hole in the canister, supplied to a combustion chamber as purge gas, and subjected to a combustion process.
The purge gas supply is controlled by a purge valve that is positioned between the canister and intake path. When the purge valve operates, causing the negative pressure in the intake path to work on the canister, the evaporative fuel purge from the canister is promoted. Consequently, the adsorption capacity of the canister is restored to normal. The adsorption capacity of the canister is limited. When the amount of the introduced evaporative fuel is beyond the adsorption capacity, the evaporative fuel overflows the canister. If the evaporative fuel overflows, the emission deteriorates. Therefore, the rate of purge gas flow into the combustion chamber should be set as high as possible to prevent the evaporative fuel adsorption amount from exceeding the canister capacity. Meanwhile, the purge gas containing evaporative fuel may cause disturbance by varying the air-fuel ratio. It is therefore necessary that the purge gas flow rate be set so as not to adversely affect drivability.
Various purge control techniques were proposed. A prior art disclosed, for instance, in Japanese Patent Laid-open No. Hei 6-26409 determines a learning correction value for the amount of fuel supply from a feedback coefficient for air-fuel ratio feedback control and increases the purge gas flow rate setting with an increase in the frequency with which the learning correction value is updated, thereby making it possible to purge a large amount of evaporative fuel while minimizing the air-fuel ratio discrepancy.
The internal combustion engine performs a fuel cut to shut off the entire fuel supply, including the supply of purge gas, during the time interval between the instant at which the upper-limit engine speed is reached and the instant at which the engine speed lowers to an appropriate level or during the time interval between the instant at which the accelerator pedal is released and the instant at which accelerator pedal is depressed. When a fuel cut is performed, fuel injection comes to a stop so that air-fuel ratio feedback control is temporarily stopped. Thus, air-fuel ratio discrepancy is likely to occur after recovery from the fuel cut. Therefore, when the purge gas supply is to be resumed upon recovery from the fuel cut, it is necessary to control the purge gas flow rate in such a manner that air-fuel ratio discrepancy does not adversely affect drivability.
When the evaporative fuel is no longer purged due to a fuel cut, the above prior art resets the learning correction value update count. The purge gas flow rate then reverts to a basic value so that the purge gas flow rate setting is lower than when the fuel supply amount is learned to a considerable extent. However, if the purge gas flow rate decreases upon each fuel cut in a situation where a fuel cut is performed frequently, the evaporative fuel may not properly be purged out of the canister so that the evaporative fuel eventually overflows the canister.